Summer Solstice and Our Wobbly World

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On a clear night in the Northern Hemisphere, look up at the sky
and, over a period of hours, you'll notice the stars rising in
the east and setting in the west ... all except one, Polaris, the
Pole Star.

It has the unique responsibility of marking, to within a few
fractions of a degree, the north celestial pole. This grand title
is the point in the sky toward which the Earth's axis of rotation
is directed.

While Earth spins on its axis, which pops out at the north and
south geographical poles (that are at different locations to the
magnetic poles), it's hurtling through space in an annual orbit
around the sun.

If we measure the angle of the rotation with respect to the plane
of the orbit around the sun, we see it's tilted 23.5 degrees from
vertical.

The tilt of the axis is responsible for the seasons we experience
as the northern and southern hemispheres are alternately
presented toward the sun throughout the year.

Today (June 20) the northern hemisphere is pointing directly
toward the sun and so the Northern Hemisphere experiences the
Summer Solstice -- or the 'longest day' -- whereas the Southern
Hemisphere is plunged into winter and experiences the Winter
Solstice and the 'shortest day.'

As a Northerner, I can be assured that the months of June/July
will be my summer months and should, in theory, be nice and warm
(although being British I could probably argue that point).
However, this won't always be the case as the axis of Earth's
rotation wobbles like a great celestial spinning top, completing
one 'wobble' in 25,772 years.

As the axis changes its orientation -- or to use its correct
term, the 'precession of the axis' -- the role of pole star
passes to another. If you could hop in a time machine and whiz
forward a few thousand years Polaris will be moving around with
the other stars and a different one will be standing sentry.

In AD 3000, the star Gamma Cephei will be the pole star followed
by Deneb in AD 10000 and Vega in Lyra will be 4 degrees away in
AD 14000.

In the Southern Hemisphere, there is currently no bright star
marking the south celestial pole; its nearest is a rather faint
star called Sigma Octantis. This will change though, in about
3700 years time Omega Carinae will become the South Pole star,
followed by Delta Velorum in AD 9200 and in AD 14000 the bright
star Canopus will be within a few degrees.

This constant changing position of the celestial poles actually
scribes a circle in the sky which takes 25,772 years to complete,
but the precession of the axis also effects the seasons.

If we were to mark the position of the Earth in its orbit at the
precise moment of the Summer Solstice, i.e., when the north pole
is pointing directly at the sun, then by the time the Earth has
completed one orbit and returned to this point, its axis will
have precessed a tiny amount and not be pointing directly at the
sun. In effect, the moment of Summer Solstice will have occurred
a little earlier by 20 minutes.

This isn't particularly noticeable from one year to the next, but
over many years it causes the seasons to very slowly shift their
position until in 25,772 years when they are back to the way they
are today.